This appears to be related to the use of part.transform.findChildren. In the editor, other parts can be children of part transforms (while in flight they are distinct, non hierarichal parts, joined only by joints).

Not the only place where part.transform.findXXX is used; should probably clean up those cases as well.

And on a related note -- also need to clean up cases where transforms are deleted. Instead of using delete immediate, move to using a regular delete, but un-parent the transform first (so that it is no longer 'found' when looking through the model transform/hierarchy).